The present invention relates to an electro-photographic image forming device; and, in particular, it relates to an electrophotographic image forming device, such as a printer, a copying machine and a fax machine, in which a toner image is transferred to an intermediate transfer means, and the toner image on the intermediate transfer means is then transferred onto a recording medium. Further, the invention relates to an intermediate transfer element and an electrophotographic image forming method.
An electrophotographic image forming device operates to form a latent image on a photosensitive body, to develop the latent image on the photosensitive body and to transfer the developed image onto a recording medium, such as paper. Further, it is well known that, when transferring the developed image to the recording medium, an intermediate transfer element (an intermediate transfer means) is employed. Namely, before, transferring the toner image, that has been formed on the photosensitive body, onto the recording medium, the toner image is first transferred onto the intermediate transfer means, and, thereafter, the toner image on the intermediate transfer means is transferred onto the recording medium to form a picture image. In particular, because a multi-color picture image can be easily formed with this type of operation, an intermediate transfer element is used for a color picture image forming device, such as color copying machine and a color printer.
There are a variety of advantages in the use of such an intermediate transfer element. In particular, if an elastic layer is provided on the intermediate transfer element, possible damage to a photosensitive body can be reduced, which damage can be caused when carriers of developing agent are caught at a nip portion of the photosensitive body, so that this technique is effective for prolonging the life of the photosensitive body. JP-A-8-160763 (1996) discloses such an intermediate transfer body. Further, JP-A-9-15987 (1997) proposes to use a belt-shaped delivering means for delivering the recording medium and to provide projections of 1-6 mm at the bottom side of the belt for separating the recording medium.
In the present day, the rigidity of the paper used as a recording medium tends to be weak and the radius of curvature of the drum which the paper contacts tends to larger. For this reason, separation of the paper from an intermediate transfer drum becomes difficult when paper having a weak rigidity is used, with the result that the paper may wind around the intermediate transfer drum so as to cause an undesirable jamming.
In particular, thin papers tend to be used these days in view of environmental considerations and conservation of natural resources, and so there is a strong demand that even paper having a low rigidity have the quality of being easily separated. With regard to the size of paper sheets, as the size of the paper sheets increases, the radius of the intermediate transfer body (an intermediate transfer drum) increases, which makes separation of the paper sheets difficult and resultantly limits the size and kinds of the paper sheets that can be used. Further, with regard to the direction of delivering of the paper sheets, since the rigidity of the paper sheets in the lateral direction with regard to the paper making direction is small, the direction of delivering of the paper sheets is also restricted.
On the one hand, a variety of paper separation measures have been proposed. However, there are few measures which can be employed in practical use. For example, with a measure in which decharging is performed after transference, such as by an AC charger, to remove transferred electric charges and to reduce the adsorption force of the recording medium, such as paper, to the intermediate transfer body, when the removal of the adsorption force is insufficient, the paper separation is achieved primarily by the rigidity of the paper and the curvature of the drum.
A variety of mechanical separation methods are also known, such as catching the top edge of the paper with a claw-shaped member, and separating the paper from the back face thereof by making use of a vacuum adsorption and adhesive member, for example. However, a sufficient performance can not be obtained using such measures, because, for example, with regard to a drum shaped intermediate transfer body, since the recording medium, such as paper, electrostatically adsorbs to the drum in response to a transfer voltage, the provision of another separating means is required.
Further, with one of the mechanical separation methods, involving catching the top edge of the paper sheet with a claw-shaped member, if no separation (clearance) between the top edge of the paper sheet and the intermediate transfer body is present, the claw-shaped member can not catch the paper, so that again, it is difficult to separate a sheet of paper having low rigidity. If a claw-shaped separation mechanism is provided while limiting the kind of paper, the size of the printable region of a picture image at the top edge portion of the paper is limited, because the top edge of the paper sheet is to be caught by the claw-shaped member.
Further, an additional inconvenience may be caused in that the separation claw member may touch the picture image region, such as on the intermediate transfer body and the paper sheet, and disturb the toner image, whereby unneeded toner that adheres on the separation claw-shaped member may contaminate the face of the paper sheet or the separation claw-shaped member may touch the intermediate transfer body so as to damage the surface of the intermediate transfer body.
Still further, with the other mechanical separation method, by separating the paper from the back face thereof using such techniques as vacuum adsorption and an adhesive member, it is extremely difficult to provide a mechanism which achieves a stable separation performance for a variety of kinds of paper during the life of the machine. Further, such a separation mechanism requires a complex structure which raises the cost thereof and makes the machine less desirable for practical use.
An object of the present invention is to enable printing even under poorly separable condition in which it is difficult to separate the paper from the intermediate transfer body. More specifically, an object of the present invention is to provide an image forming device in which an intermediate transfer body having a simple structure and a low cost can be realized, and which is free from such restrictions as small rigidity of thin paper, the size of the paper and the paper making direction, and which exhibits a stable paper sheet separation performance for a long life time.
In order to achieve the foregoing objects, in an image forming device of the present invention in which a toner image on a photosensitive body is transferred onto an intermediate transfer means and the toner image on the intermediate transfer means is further transferred onto a recording medium, minute projections having a height of more than 20 xcexcm, but less than 60 xcexcm, are provided on the surface of the intermediate transfer means.